266 SUSCEPTIBILITY AND IMMUNITY. 
covery from an attack of an acute infectious disease. But the idea 
that during such an attack an antidote to the disease poison is de- 
veloped in the tissues is yet so novel, and the experimental evidence 
in support of this view is of such recent date, that it would be pre- 
mature to accept this explanation as applying to immunity in gene- 
ral, It seems difficult to believe that an individual who has passed 
through attacks of measles, mumps, whooping cough, scarlet fever, 
small-pox, etc., has in his blood or tissues a store of the antitoxin of 
each of these diseases, formed during the attack and retained during 
the remainder of his life, or continuously produced so long as the 
immunity lasts. Moreover, in those diseases to which the experi- 
mental evidence above recorded relates—diphtheria, tetanus, pneu- 
monia—as they occur in man, no lasting immunity has been shown 
to result from a single attack, and in this regard they do not come 
into the same class with the eruptive fevers and other diseases in 
which a single attack usually protects during the lifetime of the in- 
dividual. 
In those instances in which acquired immunity has been shown 
to be due to the production in the body of the immune animal of an 
antitoxin, it is still uncertain whether there is a continuous produc- 
tion of the protective proteid, or whether that formed during the 
attack remains in the body during the subsequent immunity. The 
latter supposition appears at first thought improbable ; but when we 
remember that the protective proteids which have been isolated by 
Hankin from the blood and spleen of rats, and by Tizzoni and Cat- 
tani from the blood of animals made immune against tetanus, do 
not dialyze, it does not seem impossible that these substances might 
be retained indefinitely within the blood-vessels. On the other hand, 
the passage of the tetanus antitoxin into the mother’s milk, as 
shown by Ehrlich’s experiments upon mice, indicates a continuous 
supply, otherwise the immunity of the mother would soon be lost. 
The writer has obtained (May, 1892) experimental evidence that 
the blood of vaccinated, and consequently immune, calves contains 
something which neutralizes the specific virulence of vaccine virus, 
both bovine and humanized. Four drops of blood serum from a calf 
which had been vaccinated two weeks previously, mixed with one 
drop of liquid lymph recently collected in a capillary tube, after con- 
tact for one hour was used to vaccinate a calf; the same animal was 
also vaccinated with lymph, preserved on three quills, which was 
mixed with four drops of serum from the immune calf and left for 
one hour. The result of these vaccinations was entirely negative, 
while vaccinations upon the same calf made with virus from the 
same source, and mixed with the same amount of blood serum from 
a non-immune calf, gave a completely successful and typical result. 
